1. Field of the Invention
The present invention relates to duplexer packages and more particularly, to duplexer packages using surface-acoustic-wave (SAW) band-pass filters.
2. Description of Related Art
In recent years, down-sized versions of mobile communication apparatuses typified by handy phones have been rapidly developed and, therefore, the parts for use therein have been desired to have a smaller size and higher performance. In mobile communication apparatuses, duplexers are used to separate or generate signals.
While most of the duplexers comprise band-pass filters using dielectrics, band-rejection fillers or a combination of these filters, duplexers using SAW filters are now under research and development for further down-sizing and higher performance.
The duplexer separates two surface acoustic waves having different pass bands to prevent the interference between the two surface acoustic waves. A duplexer comprising two SAW band-pass filter chips should be designed so as to prevent or reduce the interference of the filter characteristics of the two SAW band-pass filter chips with each other. To this end, the two SAW band-pass filter chips are each provided with a phase matching circuit, or at least one of the two SAW band-pass filter chips is provided with a phase matching circuit.
The phase-matching circuit may be placed in a multilayer ceramic package together with the filter chips to form a small duplexer package of about 2.4 mm in height.
In usual, the two filter chips included in the duplexer have different center band-pass frequencies (e.g., 836 MHz and 881 MHz). The filter chips characteristically show a small attenuation in the respective pass bands, and a large attenuation in the respective rejection bands so that the level of a signal becomes low.
It is also necessary to prevent the deterioration of the filter characteristics of the two filter chips when used in the duplexer. For this purpose, ideally, the impedance of each of the filter chips is infinity and the reflection coefficient thereof is about 1 in the pass band of the other filter chip.
For attaining the above characteristics, the phase matching circuit is connected to the filter chip. The phase matching circuit is generally formed of a strip-line, a condenser (C) or a inductance (L), the latter two being discrete devices.
In the phase matching circuit formed of the strip-line, the resistance increases proportionally to the length of the strip line. the increase in resistance may cause a signal transmission loss and increase in stray capacitance in a distribution constant.
The increase in the stray capacitance influences the phase circuit constant and the like to change the characteristics. Such influence is known to become greater specially as a higher frequency is used and as the material for packaging the duplexer is higher in dielectric constant.
To avoid this influence, a material having a low dielectric constant (e.g., alumina ceramic or glass ceramic) is used for the package and a conductor having a low resistance (e.g., tungsten) is used for the phase matching circuit.
When the discrete chip L or C is used, the constant of the phase matching circuit is not influenced much. However, due to the accuracy of the chip, delicate adjustment in phase matching is difficult. Further, since the size of the chip is large, the duplexer becomes of considerable height.
An example of a conventional duplexer comprising two SAW filter chips of band-pass type will hereinafter be described.
FIG. 12 is a block diagram illustrating the circuit construction of a conventional duplexer formed in a package. Referring to FIG. 12, the reference alphanumerics F1 and F2 denote SAW filter chips. Phase matching circuit P1 and P2 are inserted to prevent the interference of the filter characteristics of the filter chips.
Terminals T1 and T1' are common signal terminals, terminals S1 and S1g and terminals S2 and S2g are input/output terminals for separated signals. One of each of the above pairs of terminals (e.g., T1', S1g, and S2g) is connected to ground (GND).
Usually, the SAW filters F1 and F2 and the phase matching circuits P1 and P2 are accommodated in a multilayer ceramic package.
FIGS. 13 to 17 are schematic sectional views and perspective views showing the construction of conventional duplexer packages. Referring to FIG. 13, externally connecting terminal section 101 is located in the lowermost layer of the package, which corresponds to the terminals T1, S1 and S2 in FIG. 12.
A phase matching circuit 100 made of tungsten or the like is buried in an insulating layer 103.
In the duplexer shown in FIG. 13, the phase matching circuit 100 is inserted only between the filter F1 and the terminal T1.
One end of the phase matching circuit 100 is connected to the common signal terminals T1 in the lowermost layer via a through-hole and the other end is connected to the filters F1 and F2 via a through-hole. The filters F1 and F2 are disposed on a die-attach layer 102 serving as a face for mounting the filter chips, and are connected to a bonding terminal layer 104 via a wire 107.
The bonding terminal layer 104 is located on the surface of a layer 105 defining a cavity (hereafter referred to as a cavity layer), the surface being at the same height as the top surface of the filters, and is connected to the signal terminals S1 and S2 in the lowermost layer via an end portion of the package. The die-attach layer 102 is provided with a ground (GND) pattern, and the filters F1 and F2 are disposed on the GND pattern. On the uppermost layer, a cap 106 is disposed as a hermetic seal.
Referring to FIG. 14 showing a perspective view of the duplexer package shown in FIG. 13, the phase matching circuit 100 is formed in a layer 7. Via through-holes, an end of the phase matching circuit 100 is connected to the terminal S1' in the cavity layer 105 and the other end is connected to the terminal S2' and the terminal T1 which is located in the lowermost layer. The surface of a layer 5 is the die-attach layer, on which the filter chips F1 and F2 is disposed.
A ground (GND) layer is formed on the surface of a layer 9 and connected to GNDs in a layer 2 and a layer 6 via a through-hole and an end portion of the package.
The size of the conventional duplexer package as shown in FIGS. 13 and 14 is about 7.5 (length).times.8.5 (width).times.2.4 (height) mm.
FIG. 15 is a perspective view illustrating a conventional duplexer package wherein the phase matching circuits 100 are inserted both on a layer 7 and on a layer 8 between the filters F1, F2 and the common signal terminal T1.
Difference from the conventional duplexer package as shown in FIG. 14 lies in that the layer 8 on which the phase matching circuit 100 is formed is added. Due to the addition of the layer 8, this duplexer package is about 0.35 mm higher than that shown in FIG. 14 (2.4 mm).
FIGS. 16 and 17 show a conventional duplexer package of upside-down construction with respect to the construction shown in FIG. 13, in which the phase matching circuit 100 is formed above the die-attach layer. Compared with the duplexer package shown in FIG. 13, the insulating layer 103 can be omitted in this construction, and therefore the size of the duplexer package is about 8.5 (length).times.9.5 (width).times.1.6 (height) mm.
However, the phase matching circuit 100 disposed on the top, as it is, is ready to be affected by external radiant noise.
Accordingly, if a signal line such as GND lies close to the phase matching circuit 100, the characteristic impedance of the phase matching circuit changes and the device characteristics deteriorate. Therefore a certain space is necessary.
Conventionally, the duplexer package having the construction as shown in FIG. 16 is employed with a space of 0.6 mm or more provided above the phase matching circuit 100. After all, this duplexer package occupies the height of 2.2 mm or more in use.
As previously mentioned, the down-sizing of the parts is highly demanded for small-sized communication apparatuses, and especially there is much more need for "the lowering the part height."
The duplexer package of multilayer construction with the buried pattern of the phase matching circuit as shown in FIG. 13 can reduce the adverse effect of the filters on each other's characteristics, but it is difficult to further down-size the duplexer package or lower the height thereof because a considerable number of layers are necessary.
Also, in the construction shown in FIG. 16, since a space of a certain height is required above the duplexer package, there is a limit to the lowering of the height. Further there is a strong possibility that the filter characteristics are adversely affected by external noise.